Chemistry Reference
In-Depth Information
Equation (1.4) is a very important relationship since it allows the
calculation of
K
b
or
K
a
if the other is known. It also follows that the
strengths of acids and their conjugate bases are related through
K
w
. This
means that a strong acid must have a weak conjugate base and, similarly, a
weak acid must have a strong conjugate base. A moment's thought will
confirm that this must, indeed, be true. Acids and their conjugate bases are
related by equilibria, which can be thought of as giant seesaws. If one
partner of the pair is very strong and heavy, the other will be weak and light.
The same relationship applies to acid-conjugate base equilibria.
This relationship also allows chemists to be lazy and express the
strengths of acids and bases in terms of the dissociation constant for the
acid. This is particularly true when we consider the term p
K
a
.
In a similar manner to pH, the p
K
a
of an acid is defined as the
negative logarithm (to the base 10) of the dissociation constant,
K
a
; i.e.
p
K
a
log
10
K
a
This terminology allows chemists to talk loosely about the p
K
a
of acids and
bases, when what they really mean is the p
K
a
of acids and the conjugate
acids of bases. It is incorrect to say 'the p
K
a
of a primary amine is between
9 and 10', although the usage is widespread. It is more accurate to say 'the
p
K
a
of the conjugate acid of a primary amine is between 9 and 10'. This is
just another example of lecturers saying one thing and meaning another.
Another source of confusion concerning strengths of acids arises with
K
a
and p
K
a
. The term
K
a
is the dissociation constant for the ionisation of
an acid, and hence the larger the value of
K
a
, the stronger is the acid (since
the equilibrium constant lies farther to the right-hand side).
p
K
a
is the negative logarithm of
K
a
, and is used commonly because
K
a
values for organic acids are very small and hard to remember (typically
10
-5
). It follows that since p
K
a
is the
negative
logarithm of
K
a
, the smaller
the value of p
K
a
the stronger is the acid.
Consider the two carboxylic acids below:
Acetic acid, CH
3
COOH, p
K
a
4.7
Chloroacetic acid, ClCH
2
COOH, p
K
a
2.7
In answer to the question 'which acid is the stronger?', clearly it is
chloroacetic acid, since its p
K
a
is smaller. A student of organic chemistry
could even suggest that the reason is due to increased stabilisation of the
anion formed on ionisation by the electronegative chlorine atom. If the
question is asked 'how much stronger is chloroacetic than acetic?', then all
sorts of interesting answers appear, ranging from 'twice as strong' to a
